The present invention relates to adhesives. More particularly it relates to novel adhesive compositions which are curable to form strong, durable adhesive bonds and to methods of bonding.
There has been considerable interest in the general field of adhesives for use in the rapid mass production of a wide range of products. However, adhesive bonding, particularly with heretofore known high-strength durable adhesives, has been relatively slow and/or has required special equipment so as to present a substantial impediment to achieving commercially practical production.
Heretofore, acrylic-based adhesives, cured by free radical addition reaction mechanisms, have been suggested for use in applications which require fast curing and flexible adhesive composition. While these acrylic-based adhesives form flexible bonded joints which exhibit beneficial properties in some applications, they often possess unacceptable water-, solvent-, or heat-resistant qualities.
Curable epoxy resins have also been used as adhesive compositions. Unlike the free radical addition curing catalysts which are used with acrylic-based adhesives, the epoxy resins are polymerized by ionic addition mechanisms and often require high curing temperatures and long setting times. While epoxy adhesives form a tight polymer network and are characterized by durability, good adhesion, and good water, chemical, and/or heat-resistant qualities, they tend to be brittle; and long curing time renders such resins inappropriate for many industrial applications.
It is generally recognized in the art that acrylic-based adhesives and epoxy resin-based adhesives are mutually non-reactive together. For example, in the 3,684,617 patent the inventor deals with an adhesive mixture of acrylic-based monomer and epoxy resin, each of which requires its own curing agent resulting in separate curings without interbonding. Moreover, other composite mixtures have been suggested wherein the epoxy resin component of a composite adhesive mixture generally does not react at all. For example, the U.S. Pat. No. 3,994,764 deals with such a non-reactive composite adhesive. The '764 patent suggests a number of ingredients, one of which is an epoxy resin, selectively added to an acrylic-based adhesive. However, the '764 patent specifically teaches away from an interaction of the epoxy resin component.
An object of the present invention is to rovide a novel, quick-setting, room-temperature-curable adhesive, which combines the room temperature curing and quick setting features of the acrylic-based adhesives with the strong bonding qualities of the epoxy resin-based adhesives.
A further object of the present invention is to provide a novel, economical method of bonding workpieces together in a manner permitting relatively rapid production while providing a strong, durable bond.
The present invention takes two different adhesive components, one of which is an acrylic-based adhesive the other of which is an epoxy resin, which by their very nature are not expected to be reactive together, and chemically bonds them together. Each of these adhesive components has a unique set of properties, and the present invention combines those properties in the reaction wherein the composite adhesive is allowed to "cure".
As used herein, a "difunctional" molecule will refer to a molecule which has two chemically reactive groups of the same chemical functionality attached. For example, an ethoxylated bisphenol A dimethacrylate molecule has two methacrylate functional groups attached. Another example is the bisphenol A diepoxide molecule which has two epoxide functional groups attached. In this sense, trimethylolpropane [2-ethyl-2-(hydroxymethyl)-1, 3-propanediol] trimethacrylate is an example of a "trifunctional" molecule because it has three methacrylate functional groups attached, and tetramethylolpropane tetramethacrylate is an example of a "tetrafunctional" molecule because it has four methacrylate functional group attached. As used herein, a "bifunctional" molecule will refer to a molecule which has two chemically reactive functional groups of different chemical functionality attached. For example, methacrylic acid has an acrylic-based chemically reactive group and an epoxide-reactive group, attached to the same molecule. For purposes of illustrating the present invention, bifunctional molecules disclosed include molecules having two functional groups attached, wherein one functional group is epoxide-group chemically reactive and another functional group is acrylic-based chemically reactive.
It is known in the art that acrylic-based adhesives usually require the presence of a catalyst of the free radical type. It is also known in the art that suitable catalysts include the organic peroxides, organic hydroperoxides, and organic peresters, which can be exemplified respectively by benzoyl peroxide, cumene hydroperoxide, and acetyl peroxide. It is known in the art that acrylic-based materials generally contain inhibitors. It is also known in the art, that other materials can also be added to the acrylic-based adhesive composition to affect the speed of the reaction of the adhesive composition. For example, it is known that certain substances, acting as inhibitors, will interfere with the catalyst in such a way as to substantially reduce the rate of polymerization, and that certain metal salts, functioning as initiators or activators, can be added to accelerate the rate of polymerization.
As used herein, the "adhesion reaction" as illustrated in the following example wherein workpieces are bonded together comprises two steps: initial "set up", which is usually quick and often takes no more than a few minutes, and final "cure", which is the polymerization reaction between the acrylic-based and epoxide-based components of the adhesive. The "quick set" feature of the present invention is obtained during this initial set up; and the term "quick set" as herein used means that workpieces are substantially adhesively bonded together by the adhesive mixture of the present invention within a short period of time which may, for example, be in the neighborhood of ten minutes or less. The bond provided by the "quick set" is sufficient to permit some further handling of the assembled workpieces such as may be necessary for removal from an initial assembly and bonding-station or -apparatus prior to a final cure of the adhesive, thereby promoting faster and more economical production.